Comparative population dynamics of the rotifers Brachionus angularis and Keratella cochlearis

The results indicate that the two rotifer species Brachionus angularis and Keratella cochlearis have different life history strategies. Brachionus angularis appeared to be an r-strategist, showing higher reproductive, mortality and population growth rates than K. cochlearis. Additional evidence is offered suggesting that K. cochlearis is a K-strategist. Although K. cochlearis had a lower growth rate than B. angularis, this was compensated by a lower mortality rate, especially in pre-reproductive individuals. In addition, K. cochlearis allocated energy to reproduction more efficiently than B. angularis.     

Food limitation and seasonal population declines of rotifers

Summary Keratella cochlearis exhibited consistent seasonal abundance patterns during a four-year study in Little Rock Lake, Wisconsin, U.S.A. In each year, spring population peaks were followed by strong summer reductions. Concomitant with population declines, there were reductions in rotifer egg ratios. Keratella taurocephala abundance patterns were similar to those of K. cochlearis during 1984 and 1985, but not in 1986 and 1987, when spring peaks and summer declines were not apparent. However, summer declines in the egg ratio of K. taurocephala were observed during each year. The reduction in rotifer populations simultaneously with decreased egg ratios suggested that population declines were caused by food limitation. Food-addition experiments conducted in situ in small enclosures indicated that food was limiting for K. cochlearis when its populations were declining, but not during other periods of the year. Keratella taurocephala did not show a consistent response to food addition.     

An Experimental Analysis of the Acid Sensitivity of the Common Planktonic Rotifer Keratella cochlearis

An in situ mesocosm experiment was performed at neutral pH Lake O'Woods, West Virginia, to assess the impacts of acidification on the common planktonic rotifer Keratella cochlearis. This rotifer is typically replaced by K. taurocephala during the acidification of North American lakes. Despite a rapid pH reduction in this experiment (from 7.0 to 4.8 in 14 days), the abundance, mean body length and egg ratio of K. cochlearis did not decline in the acid treatment as compared to the untreated control. These results support the hypothesis that K. cochlearis is acid-tolerant, and suggest that its disappearance from acid lakes is the result of biotic interactions within the plankton, rather than the result of toxic effects of altered water chemistry.     

Regulation of rotifer species by invertebrate predators in a hypertrophic lake: selective predation on egg-bearing females and induction of morphological defences

Temporal changes in the biomass of rotifer plankton were examinedin a shallow hyper trophic lake during 1 month (April/May),when the predatory rotifer Asplanchna brightwelli develops.The abundance of herbivorous rotifers was regulated by predationfrom A.brightwelli and from the copepod Acanthocyclops robustus.The densities and fecundity rates of Keratella cochlearis andKeratella quadrata were negatively related with the biomassof predators. Stomach analyses showed that Asplanchna fed selectivelyon reproductive females of K.cochlearis, reducing the fecundityof this species. Predators induced longer caudal spines in K.quadrata,which were negatively related to the fecundity of this rotifer,suggesting a reproductive cost associated with spine production.In contrast, spine length of K.cochlearis was not related topredators, but to temperature. These results showed that predatorscan reduce rotifer densities through increasing mortality andthrough decreasing rotifer fecundity rates regardless of phytoplanktonbiomass. We also show that morphological defences of K.cochlearisand K.quadrata are induced in different ways.     

Direct Estimation of Food Consumption of Juvenile Fish in a Shallow Inlet of the Southern Baltic

The species composition and feeding behaviour of fish larvae and juveniles was investigated in a shallow inlet of the Southern Baltic. The aim of the study was to examine if there was a predatory impact of young fish on the observed annual collapse of the copepod population. Dominant fish species were herring (Clupea harengus), perch (Perca fluviatilis) and roach (Rutilus rutilus). Roach larvae preferred shallower areas, whereas young herring and perch were found in open water. Highest consumption values were always observed in the evening. By computing complete evacuation of the digestive tract in approximately 3.5 hours, daily consumption in percent of fish fresh weight was 1.3, 4.7 and 5.5% for young herring, roach and perch, respectively. The impact of young fish on the dynamics of the population of the copepod species Eurytemora affinis is negligible.     

Prey selection by Asplanchna girodi (Rotifera): the importance of prey defence mechanisms

1. Predator (laboratory-grown Asplanchna girodi and prey (several field-collected non-carnivorous rotifer species) interactions were studied in the laboratory as a function of both predator and prey densities. The clearance rates of A, girodi decreased with increasing prey density. Predator density had no effect on the feeding of A. girodi. 2. Asplanchna girodi selectively fed on Keratella cochlearis. Its clearance rates for K. cochlearis were much higher than those for Polyarthra and Brachionus. Short-spined forms (spine length less than 25 μm) showed a significantly higher susceptibility to predation than either the non-spined or the long-spined forms. 3. Large Asplanchna individuals fed selectively on reproductive females of K. cochlearis, thereby reducing the fecundity of their prey.     

Decline of Clear-Water Rotifer Populations in a Reservoir: The Role of Resource Limitation

The relative importance of and changes in resource limitation of herbivorous rotifers were assessed during the clear-water phase in the Rímov Reservoir, Czech Republic, using in situ manipulative experiments. Resource limitation was tested experimentally as the difference in population growth rate (Δr) among various experimental treatments on four occasions. The reservoir community of rotifers was exposed to three treatments: (i) control, (ii) diluted and (iii) diluted and fertilized. Significant responses to these experimental manipulations were shown by Synchaeta spp., Polyarthra spp. and Keratella cochlearis. Growth rate was usually highest during the spring rotifer maximum and decreased during the clear water phase. The highest intensity of food limitation (expressed as ‚Chlorophyll-a’ limitation) was found in Synchaeta spp. K. cochlearis had low food limitation during the spring peak, high food limitation during the second experiment and low food limitation, again, during the later experiment. In contrast, Polyarthra spp. had the same Chlorophyll-a limitation throughout the whole experimental period. Linear regression was used to estimate the relative proportion of Δr variability explained by Chlorophyll-a concentration and rotifer density in all of the experiments. Chlorophyll-a concentration explained 89, 97 and 92% of the resource limitation in Synchaeta spp., Polyarthra spp. and K. cochlearis, respectively. The proportion of variability explained by rotifer density-dependent factors was lower: 60% for Synchaeta spp. and 68 % for Polyarthra spp.     

Regulation of Rotifer Community by Predation of Cyclops vicinus (Copepoda) in the Římov Reservoir in Spring

The structuring role and predation impact of Cyclops vicinus on the rotifer community was studied using in situ enclosure experiments in Římov Reservoir during the spring of 2000. Seasonal changes in abundance and birth rates of the rotifer species Synchaeta lakowitziana , Polyarthra spp., Keratella cochlearis and Kellicottia longispina were related to predation pressure, i.e. selectivity and cropping rates of C. vicinus . All four rotifer species were found to be consumed by C. vicinus whose predation rates showed a marked succession through the spring period and corresponded to changes in the relative abundance of the pelagic species. The highest predation rates exhibited by C. vicinus were on Synchaeta lakowitziana and Polyarthra spp., being 11.3 and 6.4 ind Cyclops ^–1 day^–1 respectively. The activity of the predator switched among three types of preferred food during the spring period. The most preferred, Synchaeta, was replaced by Polyarthra after Synchaeta became extinct and these species were replaced by Keratella when both soft‐bodied forms were not available in sufficient quantities. Loricated species (K. cochlearis and K. longispina) were consumed at a slower rate. We calculated the proportion of rotifer production cropped day^–1 in order to estimate the predation impact of C. vicinus , which ranged from 11.6 to more than 100% over time and rotifer species. This shows that relative predation impact can be high not only on species most strongly selected, but also on species which are not selected. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparative analysis of rotifer community structure in five subtropical shallow lakes in East China: role of physical and chemical conditions

Worldwide, there have been few comparative studies on rotifer communities in subtropical lakes. We studied changes in rotifer community structure over 1 year and its relationship to several physicochemical variables in five subtropical shallow lakes in East China, covering a nutrient gradient from mesotrophy to moderate eutrophy. In these lakes, the genera Brachionus, Lecane, and Trichocerca dominated the rotifer species composition, and Polyarthra dolichoptera, Keratella cochlearis, Filinia longiseta, T. pusilla, and Anuraeopsis fissa were the dominant species. With increased nutrient loading, total rotifer abundance and species dominance increased, indicating that rotifer abundance might be a more sensitive indicator of trophic state than species composition. Comparative analyses of the six rotifer community indices calculated in this study and redundancy analysis (RDA) revealed that the two slightly eutrophic lakes and the other two moderately eutrophic lakes exhibited a high degree similarity in community structure. This suggests that the trophic state of a lake determines the rotifer community structure. In contrast, in the two moderately eutrophic lakes, the mass ratios of TN:TP and the contents of TP suggested N-limitation and cyanobacteria dominance in phytoplankton communities might be possible. In these lakes TN played a more important role in shaping the rotifer community according to stepwise multiple regression and RDA. RDA analysis also suggested that rotifer species distribution was strongly associated with trophic state and water temperature, with water temperature being the most important factor in determining seasonality.     

Urban rotifers: structure and densities of rotifer communities in water bodies of the Poznań agglomeration area (western Poland)

The rotifer fauna of 19 mostly small water bodies (natural and artificial ponds, clay-pits and pools) in Pozna was studied on four occasions during 1996–98 to determine the suitability of urban areas for rotifer habitats. Rotifers were present in all the water bodies studied, with 114 species in 39 genera found, representing ca. 25% of all rotifers recorded from Poland. Mean diversity was 10 spp (range 1–36). Most common were: Brachionus angularis and Keratella cochlearis (spring), Colurella uncinata, Lecane closterocercaand Lepadella patella (summer) and L. closterocerca and K. cochlearis (autumn). Rotifer densities (1-1503 ind l^–1), Shannon's diversity (H 0.00-3.71) and dominant species differed in different water-bodies. The index of percentage similarity of community showed strong differences in qualitative structure of rotifer assemblages. The different types of water habitats, both the existing or the newly created in towns, may explain the relatively high diversity of rotifer communities observed in the urban areas studied.     

Abundance and distribution of pelagic rotifers in a cold,deep oligotrophic alpine lake (Königssee)

Rotifer community structure of the oligotrophic, alpine lake KGnigssee was investigated from October 1982 to October 1983. Twenty different species were found, including several cold-stenothermal and oligotrophic species. Polyarthra vulgaris/dolichoptera, Kellicottia longispina and Keratella cochlearis were the most abundant species throughout the year, comprising 90% of the total rotifer community (2.8 × 10⁶ individuals m^–2, maximum). The remainder of the rotifer community was represented by as many as eight species which occured for a short time in summer and autumn. The rotifer community was limited to the upper 50 m (85–100%) in which most (60–80%) of the rotifers preferred the trophogenic layer (0–20 m). Species-specific depth preferences were observed. Rotifer abundance and distribution are discussed in terms of the specific environmental conditions in Königssee. The rotifer community of Konigssee is compared to that of Lake Constance in its former oligotrophic state.     

Relationships Between Rotifers,Phytoplankton and Bacterioplankton in the Corumbá Reservoir,Goiás State,Brazil

This study evaluates the relationships between rotifers and phytoplankton and rotifers and bacterioplankton in a tropical reservoir. Fourteen stations in the reservoir were sampled, including in its arms and tributaries, in the dry and rainy seasons. The highest rotifer density was found in the dry season, mainly in the upper and intermediary stretches of the reservoir. Brachionus calyciflorus, Polyarthra vulgaris, Keratella tropica, K. cochlearis, K. americana and Pompholyx complanata were the most abundant species. Densities of B. calyciflorus and bacteria were significantly correlated. On the other hand, there was an inverse relationship between P. vulgaris and bacteria. Diatoms (Bacillariophyceae) were observed to be associated to K. americana. We suggest that the rotifer populations play a part in microbial and herbivory food webs.     

Changes in Rotifer Species Composition and Abundance along a Trophic Gradient in Loch Lomond,Scotland, UK

Loch Lomond is the second largest body of freshwater in Great Britain. It is a long, narrow lake (36.4 km long, 8.8 km wide). The northern basin is fjord-like and surrounded by a mountainous, base-poor, rocky catchment. In contrast, the southern basin is much broader and shallower with a mainly lowland, calcareous, agricultural catchment. This causes a trophic gradient along the length of the loch that runs from the oligotrophic northern basin to the more mesotrophic southern basin. Rotifer samples were collected at monthly intervals between May and October 2002 at three locations along the length of the loch. More than 12 species were found, the commonest of which were Keratella cochlearis (Gosse) and Trichocerca stylata (Gosse). Although the species composition of the rotifer community varied very little among the sites, rotifer abundance increased markedly from north to south, apparently reflecting the trophic gradient along the length of the loch. The results suggest that rotifer abundance may be a more sensitive indicator of trophic state, and changes in trophic state, than species composition.     

Use of fluorescently labelled algae to measure the clearance rate of the rotifer Keratella cochlearis

1. Fluorescently labelled algae (FLA) were used to measure clearance rates of the rotifer Keratella cochlearis. The freshwater algae Chlorella vulgaris and Stichococcus bacillaris were labelled with a fluorescent dye, 5-(4,6-dichlorotriazin-2-yl) aminofluorescein (DTAF), following a modified staining procedure. 2. Keratella cochlearis ingested both algal species. Clearance rates on tracer foods varied between 2.4 and 6.9 μl ind^?1 h^?1, which are comparable with those determined using other techniques. 3. The main drawback of the FLA technique was that only a little more than one-third of the total amount of algal cells of both C, vulgaris and S. bacillaris were well stained with the dye (DTAF), despite the use of a higher concentration of dye and a longer staining period than recommended in the literature. 4. The FLA method can be successfully applied in grazing studies involving size selection and competition for food among zooplankton. The method complements existing techniques for measuring the clearance and ingestion rates of filter-feeders.     

Seasonal fluctuation of planktonic rotifers in Azibo reservoir (Portugal)

The rotifers of the recently-commissioned Azibo reservoir were studied during two years. The community was similar to that of other portuguese reservoirs with Keratella cochlearis, Polyarthra spp., Asplanchna priodonta and Collotheca pelagica dominant, although densities varied strongly. Maximum rotifer density was 640 ind l^–1 in 1987 and 315 ind l^–1 in 1988. During the first year two density peaks were observed, while in the second year fluctuations were irregular. Phytoplankton availability was considered the main reason for the fluctuation of rotifer density, although temperature, dissolved oxygen and cladoceran interference may also have played a role. A high density of Anabaena flos-aquae might have facilitated the dominance of K. cochlearis in 1988.     

The effects of a change in the growth rate of roach, Rutilus rutilus (L.), on the biology of the fish tapeworm Ligula intestinalis (L.)

Investigations into the biology of the roach and the pathogenic tapeworm Ligula intestinalis (L.) populations at Slapton Ley, Devon were carried out between October 1982 and December 1984. Data collected from the lake since 1977 have also been re-analysed to determine how the dramatic improvement in the individual growth rate of the roach over this period may have affected the growth, maturation and life-cycle of Ligula. Only the very young roach at this site become infected, so it was possible to follow cohorts of plerocercoids of similar age through each roach year class. Results for the 1978 and 1983 year classes are presented in detail. The roach grew extremely rapidly from May to August in each year, resulting in a pronounced cyclical pattern of changes in the condition of the roach, with the lowest condition occurring in late winter and spring. The yearly increase in the roach growth rate was accompanied by an increase in plerocercoid growth rate in the 0 + roach, but not in the 1 + roach. The growth rate of the plerocercoids was very high compared to that at other sites. It is usual for the parasite index (PI) of Ligula-infected fish to be high and to increase throughout their first few years of life. At Slapton, however, a lack of multiple infections has prevented high PIs from occurring, and in recent years the growth rate of the roach has been so high that the plerocercoids were unable to maintain a high weight relative to the fish, and the highest PIs occurred in the 0 + roach throughout late winter and spring. In recent year classes, therefore, the maximum PIs and highest pathogenicities coincided with the period of lowest condition in the 0+ roach. Observations of both caged and natural populations of 0+ roach over winter showed that a significant loss of roach containing the larger plerocercoids occurred from the population. In vitro cultivation of Ligula plerocercoids showed that they were capable of maturation at weights of 0.5 g, and only 6 months after having infected the roach. The increase of the growth rate of the plerocercoids in the 0+ roach has therefore resulted in a greater proportion of these plerocercoids being capable of infecting the definitive host. As a result of the increase in individual growth rate of the roach at Slapton, the potential for Ligula transmission, as measured in terms of both their pathogenicity and maturity, has shifted from the 1 + to the 0+ roach.     

Effects of net, pump and bottle sampling on the abundance estimates of planktonic Rotifera

Eighty-seven freshwater plankton samples collected by meansof two different nets, a submersible centrifugal pump and a5.81 water bottle were compared with respect to their abilityto catch the rotifer Keratella cochlearis. Sample size, illumination(day/night), the presence of bridles ahead of the net mouths(versus unbridled nets), and different mouth diameters (0.2and 0.5 m) did not affect abundance estimates. Slight differenceswere found between the yields of pushed nets versus towed nets;these are probably due to uneven distribution of the animalsin the 0–3 m layer. Both pump and bottle volumes stronglyaffected abundance estimates (K. cochlearisl^–1 in pumpsamples=164.84 volume of water filtered^{–0 817}; K. cochlearisl^–1in bottle samples=84.74+2336.6 volume of water filtered^–1)Net sample results were always considerably higher (4.6–12.3times) than pump and bottle estimates; these differences aremost probably due to evading reactions of the rotifer.     

Seasonality,population dynamics and production of planktonic rotifers in Lough Neagh,Northern Ireland

The abundances, population dynamics and production of the rotifer community of Lough Neagh were examined for a three year period. Keratella cochlearis was the most abundant species accounting for over 40% of biomass followed by Polyarthra dolichoptera and Notholca acuminata. The mean standing crop for the rotiferan zooplankton increased in successive years (41, 51, 75 mg dwt m^–2) as did production (1037, 1322, 1417 mg dwt m^–2 y^–1). The seasonal pattern of biomass expression and production varies markedly in different years. Instantaneous birth rates tend to be lower but more consistent for the more abundant species, instantaneous death rates show periods of negative mortality indicating an inadequacy of the model employed but explicable as hatching of resting eggs. K. cochlearis as the most successful species is explained as its perennial appearance and adaptation to the low annual temperature cycle found in the lough. The population succession and the annual occurrence of species differs in each year.     

Rotifer population dynamics in response to increased bacterial biomass and nutrients: a mesocosm experiment

The impact of organic nutrients and massive addition of bacteria was followed in lake water mesocosms in a eutrophic lake. Increased DOM initiated a sequence of trophic responses indicated by rapid increases in bacterioplankton, protozoa, and algal biomass. The populations of Keratella cochlearis and Keratella quadrata showed a distinct response by rapid increase in birth rate followed by maxima of production and abundance. This succession clearly reflected the trophic position of these rotifer populations in the food chain. A reverse response was observed in Conochilus unicornis.